The present invention relates to a method for inspecting a display screen. More specifically, the invention relates to a display screen inspection method for automatically deciding the conformity of inspection results in the production line with respect to inspections for point defects and the like on display devices used in electronic equipment and other fields, such as liquid crystal panels, shadow masks, CRT panels, and plasma displays.
As a method for reading the screen displayed on a display device, there has been provided, as a primary method, one using two-dimensional CCD area sensors.
Generally, pixels arrayed in columns and rows of two-dimensional sensors, and pixels arrayed in columns and rows of a display device are associated with each other (hereinafter, unless otherwise specified, pixels of the display device will be referred to as "display pixels", and pixels of the sensors will be referred to as "sensor pixels"), where the pixel arrangement is such that a plurality of sensor pixels are associated with one display pixel.
This is explained by taking a case of a display device of a liquid crystal panel consisting of 640.times.400 pixels as an example. A liquid crystal panel includes display operating portions and display non-operating portions of the pixels. Point defects in the dynamic operating inspection for these display operating portions can be classified into dark dots, which are a group of display pixels that will not operate for display in the displaying state of the liquid crystal panel although they are originally intended to be a display operating portion, and bright dots, which are a group of display pixels that will operate for display in the non-displaying state although they are originally intended to be a display non-operating portion. For example, in an automatic inspection for this, when three sensor pixels are assigned to one display pixel, approximately 2000 pixels are required in the row direction.
As an example of the conventional method, there has been adopted a method in which the position where the image is focused on the CCD surface is shifted by moving the lens system of CCD area sensors in advance so that contrast differences between display operating portions and display non-operating portions of the display device will not occur. Hereinafter, the state in which the image is focused on the CCD surface is referred to as a "focused state," and the state in which the image cannot be focused is referred to as a "defocused state."
Now consider a display pixel 100 as shown in FIG. 10A. Assume that a dark-dot defect 101, as shown in FIG. 10A, is present on this display pixel 100. FIG. 10B shows density distribution data in the focused state at ten display operating portions on a one-dimensional reference line 102 on the display pixel 100, where display operating portions 103 and display non-operating portions 104 are part of them. Also, the dark-dot defect 101 is present in this density distribution data. FIG. 10C shows density distribution data in the defocused state of the display pixel 100, where a dark-dot defect portion 105 (a defocused state of the dark-dot defect 101) is present in a background density 106. This background density 106 is a defocused state part of the display operating portion 103 and the display non-operating portion 104.
This conventional method is frequently used for large degrees of point defects, being an effective method. A contrast difference between display operating portions and display non-operating portions in this case refers to a contrast difference in the sensor pixel that has received light.
However, the aforementioned conventional method raises the following issues.
Making a defocused state, as described above, would result in reduced contrast differences between point-defect portions and normal portions, such that the method could not maintain enough detection ability for small degrees of point defects. In other words, making a defocused state until the difference between the display operating portions and the display non-operating portions of the display device is nullified would cause the contrast differences of small degrees of point defects to be eliminated. Thus, the conventional method has a disadvantage of being incapable of detecting such small degrees of point defects.
In some cases, it is necessary to decide whether or not the position of a display operating portion of the display device is normal. However, as described above, making a defocused state would make it difficult to decide where the proper position of the display operating portion should be.
In other cases, it is necessary to decide whether or not the size of a display operating portion of the display device is normal. However, as described above, making a defocused state would make it difficult to decide how the proper size of the display operating portion should be.